Laser printers are widely used to reproduce information or images on photosensitive media such as paper or film. One application for a laser printer is in the medical field to reproduce continuous tone x-ray images on film. Typically, the laser printer is a digital laser printer which converts an array of numerical data into an image. Each element of the image is a number which represents the desired density for one pixel of the image. The image can be derived from a medical imaging modality (MRI, CT, US, PET), a digitized x-ray film, a scanned storage phosphor having a latent x-ray image, or an x-ray image archived in optical or magnetic storage devices.
A laser printer varies light intensity to control exposure and therefore density of the laser print. The objective is to print predictable densities with minimum increments to produce a nearly continuous gray scale over the desired density range. Typically, control is a two stage process. One stage (limit control) sets the maximum exposure and the other (print control) adjusts exposure within that limit. Prints are made with different limit control settings to find the setting for the desired maximum density. A calibration print is then made which has a range of print control settings and the limit setting for desired maximum density. This provides density as a function of print control setting for that limit, making it possible to create a table which will convert desired density to print control setting.
A traditional technique for calibrating a laser printer is as follows:
First, a calibration page is printed with a limit setting to produce the desired maximum density and a full range of print settings. The next step is to determine whether this is the desired limit setting by visually inspecting the printed page. The normal objective is to find the minimum exposure required to print the full range of desired densities. The lower the limit setting, the more nearly continuous the gray scale in the printed film. The process of printing and adjusting the maximum limit setting is repeated until a desired limit setting is determined.
Next, a page is printed with the limit system setting selected and with a subset of print system settings which cover the full range of print settings. The resulting densities of the printed page are then measured and a print setting to density table created for the full range of print settings. An output lookup table that can be used to set exposure to produce the desired density for any digital image value is created using the print setting to density table. Thereafter the laser printer prints pages with this output lookup table to produce the desired densities while the same maximum exposure is appropriate. However, if maximum exposure is changed the calibration process must be repeated.
A problem which arises with this calibration technique is that calibration data is specific to a particular limit control setting. If that setting needs to be changed the entire process of successive prints to find the desired limit control setting for maximum density and calibration must be repeated. Also, if different users want different maximum densities each requires separate calibration. Such repeated calibrations is inefficient, costly and nonproductive.